1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and particularly to a dry-etching method which dry-etches a multi-layer film including films of different kinds.
2. Description of the Background Art
With finer patterning and higher integration of a semiconductor device, there is a further need for high control performance on the final shape, reduction of foreign particles, improvement in productivity and cost reduction in dry-etching technique, which is one of fine-patterning techniques.
One of dry-etching systems is RIE (Reactive Ion Etching) device employing a reactive gas. One example is shown in FIG. 31. The device has a so-called parallel-plate type structure in which an anode 30 and a cathode 31 opposite to each other are provided within a chamber 20. A wafer 25 is mounted on cathode 31. The pressure inside chamber 20 is reduced by a dry vacuum pump 26 to approximately 10.sup.3 Torr. CF.sub.4 is used as the reactive gas, for example. CF.sub.4 discharges due to a high frequency (rf) 28 and a plasma occurs. In the plasma, CF.sub.4 exists as CF.sub.3+ ions. The CF.sub.3 ions are accelerated by a space-charge region formed in the vicinity of a sample. The accelerated ions collide against the sample and hence a chemical reaction will occur between a reactive species absorbed on the sample and the sample, thereby etching the sample anisotropically. Other than such a gas of the CF system, there are various reactive gasses such as Cl.sub.2 and HBr.
Typically, it is necessary to use a different etching system depending upon the kind of the film to be etched, since process parameters such as pressure and power are greatly varied depending upon whether the film to be etched is an oxide film, a polysilicon film or an aluminum film and since foreign particles should be reduced.
For a gas of the CF system, carbon contamination will occur since the gas contains carbon atoms. Thus, NF.sub.3 is known as a gas which does not contain carbon atoms and is capable of etching both an oxide film and a polysilicon film. This is because NF.sub.3 produces radical of fluorine atoms. This also allows NF.sub.3 to be used as a gas for cleaning the inside of the chamber of an etching system used to etch polysilicon films.
However, when a polysilicon film is patterned by NF.sub.3, for example, the radical of fluorine atoms etches a sidewall of the pattern. As a result, side-etching will occur and a desired pattern cannot be obtained. Furthermore, when NF.sub.3 is used as the cleaning gas, it etches products adhering to the interior wall of the etching system, sometimes undesirably producing foreign particles.
Such a phenomenon is considered as follows: in the RIE provided with parallel-plate type electrodes, NF.sub.3 dissociates as expressed by the following chemical formula: EQU NF.sub.3 .fwdarw.NF+2F
or the following chemical formula: EQU NF.sub.3 .fwdarw.NF.sub.2 +F.
As a result, the radical of fluorine atoms becomes dominant, causing side-etching or peeling off of the products on the interior wall of the chamber.
Generally in anisotropic etching, a sidewall protecting film is formed on a sidewall of a pattern. The protecting film protects the sidewall and a desired pattern is formed. Accordingly, in order to prevent side-etching or the like, it is required to protect the sidewall to overcome the action of the radical of fluorine atoms or to constrain the action itself of the radical of fluorine atoms.
NF.sub.3 contains nitrogen atoms. By forming a nitride film containing the nitrogen atoms on the sidewall, a strong protecting film can be formed. For this purpose, it is essential that NF.sub.3 completely dissociates into nitrogen atoms and fluorine atoms as shown in the following chemical formula: EQU NF.sub.3 .fwdarw.N+3F.
Generally, dissociation of a reactive gas greatly depends on the gas pressure and plasma density within the chamber of an etching system. It is known for the parallel-plate type RIE that electron density or plasma density is approximately 10.sup.10 /cm.sup.3, pressure of the reactive. gas is at least 100 mTorr and ionization degree defined as the degree of ionized molecules is 10.sup.-4 to 10.sup.-30
However, as described above, in the parallel-plate type RIE, the nitrogen atom exists bound with fluorine atoms such as in the form of NF or NF.sub.2. That is, this system structure could not supply energy enough to completely separate NF.sub.3 into nitrogen atoms and fluorine atoms.
Conventionally, when films of different kinds are to be etched in a parallel-plate type RIE, it has been required to use a different etching system depending upon the kind of the film to be etched in order to reduce foreign particles within the chamber. As a result, the number of steps employed is increased, prolonging the process period and increasing production cost. Also, every time a wafer is inserted into or ejected from the etching system, foreign particles adheres onto a pattern and hence yield is degraded. Furthermore, if NF.sub.3 is applied as a reactive gas which does not contain a carbon atom in order to prevent carbon contamination, NF.sub.3 cannot dissociate sufficiently. As a result, a sidewall of a pattern is etched and hence a desired pattern cannot be obtained.